Time to divide the iris

Victoria Falls is a bearded iris that can bloom up to three times a season with plenty of space, sunlight, and plant food. CINDY MCNATT/ORANGE COUNTY REGISTER

August is the month that we divide the iris. There is no questioning the reason why. After about two years – three, tops – iris clumps will grow too thick and the blooms aren't as spectacular as they used to be.

Why divide irises now? Chances are they aren't blooming this time of year, they can tolerate summer disturbance, and dividing now will give them plenty of time to settle in before the winter months.

“Tall bearded iris really need it most,” said John Schoustra, breeder at Greenwood Gardens in Somis. “They get impacted just like wisdom teeth.” Plus, they are just plain easy to work with.

Since irises are shallow-rooted, use a sharpened shovel to dig up wide clumps. Try to include some of the roots by digging at least 6 inches deep.

Use a hose to wash the soil off your clump so you can get a good look at the health of each rhizome.

I like to use a serrated bread knife to divide the root zone of perennials such as iris plants. You can cut the clump into smaller ones and then use the knife to cut apart individual rhizomes. Any part of the clump of rhizomes that has no leaves, is cork-like and hollow, rotted, black or squishy can be thrown in the trash.

What should be left are plump, firm, young rhizomes with plenty of leafy tops. These are usually on the perimeter of the old clump and are the rhizomes you are going to replant.

“Iris really like a new spot,” Schoustra said. “It's not that they've used up the soil they were growing in, it's just that they respond better to a new location.” Schoustra suggests throwing a handful or two of gypsum into the soil for each rhizome you are replanting.

“Gypsum is good for all types of garden soils,” he said. “It unlocks the nutrients that tap water locks up and helps leach salts from the soil.”

Pro tip: Schoustra applies gypsum to the entire garden, including his lawn, two times a year – right before Super Bowl and again before Labor Day.

Replant iris rhizomes root down and leaf side up in full sun. Schoustra says to make sure the tops of the rhizomes are covered with a quarter inch of soil. Irises will pop above the soil surface later all on their own.

Irises like lots of room to grow, so don't plant them too close together. Leave 6 or so inches between each plant. Begin to feed in spring.

You are likely to have many more divisions of iris than you have room for. These are great plants to give away to friends and new homeowners, or swap with your garden pals.

And look for new iris plants to hit the nurseries sometime this month. Schoustra said he will begin his annual shipments to Roger's Gardens in a week or so.

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